Many industries utilize as a processing fluid, among other things, a contaminant-free inorganic mineral acid such as sulfuric or nitric acid. Sulfuric acid, for instance, is considered the commodity chemical produced in highest quantity worldwide. The filtration of such processing fluids is a formidable task, replete with a number of problems. Inorganic mineral acids, by their nature, are highly corrosive, polar substances; properties that have made it difficult to design systems which offer efficient, economical, integrity testable filtration in a point-of-use filter.
In critical processes involving fluid filtration, e.g., the filtration of parenteral pharmaceuticals, ultra pure water, and microelectronics processing fluids, one must achieve the highest possible assurance of filter integrity and contaminant removal efficiency. An example of an industry-accepted non-destructive integrity test is the "forward flow test." A forward flow test provides an objective and quantifiable method of determining filter integrity during operating conditions (in situ). Unless integrity testing is performed under operating conditions, there can be no assurance that the filter functions as intended and as required. In order to perform an in situ forward flow test, the filter must spontaneously and completely wet with the fluid to be filtered. Thus an in situ integrity testable filter for inorganic mineral acids must not only remain inert to the corrosive action of the acid, it must also spontaneously and completely wet with the acid.
Prior to the present invention, no one has been able to develop a polymeric filter medium that is both resistant to the corrosive environment of a concentrated inorganic mineral acid and capable of spontaneously and completely wetting with the acid, thereby making it in situ integrity testable. The need for such filter devices has recently become critical in a number of industries. For instance, there is a need for in situ integrity testable filters for inorganic mineral acids in the integrated circuit industry. During the manufacture of advanced microelectronic devices sulfuric acid is utilized as a processing fluid, usually at high concentrations. For purposes of this disclosure, the term concentrated inorganic mineral acid is intended to include sulfuric acid having a concentration of about 85% and greater, or nitric acid having a concentration of about 70% and greater. Concentrations of 96% sulfuric acid are common and typical in the integrated circuit industry.
Integrated circuit production involves hundreds of steps in which silicon wafers are repeatedly exposed to processes such as lithography, doping, and deposition of materials. In addition, numerous etching and cleaning steps are carried out by direct contact of the wafer with concentrated sulfuric acid. Contamination alters the semiconductive nature of the silicon or disturbs the intended circuit design, thereby reducing the yield of integrated circuits. It is, therefore, imperative that the sulfuric acid be free from particulates and contaminants, and it is highly desirable that the integrity of the filters be tested in order to prevent the production of manufactured items with a high degree of defects. Consequently, this industry has developed some of the most stringent requirements concerning the purity of processing fluids. Filtration systems are employed at point-of-use as well as at the point-of-distribution of the sulfuric acid.
A point-of-use filter processes fluid which is to be utilized immediately in a localized manufacturing step. Point-of-use filters used for sulfuric acid must remove micro particulates without themselves adding contaminants, i.e., exhibit low levels of ionic and total organic carbon (TOC) extractables. In addition, the effluent sulfuric acid from point-of-use filters must approach a high level of purity as quickly as possible, for both economic and environmental reasons.
Concentrated sulfuric acid is highly corrosive. All components of a filter for sulfuric acid must be resistant to the chemical action of the acid during short term as well as long term exposure, so that the filter does not leach ionic or organic extractables. A colorimetric assay, wherein the transference of color to the filtered sulfuric acid is quantified, is a useful method for detecting if a filter leaches unacceptable levels of extractables.
For example, the American Public Health Association (hereinafter referred to as APHA) has published a standard method for the examination of water and waste water based upon the visual comparison of the sample with commercially available Hazen Color Discs. This method is also utilized by industry for the examination of various chemical reagents as described in ASTM standard method D 1209-69. For example, the American Chemical Society Specification for sulfuric acid (95% to 98%) is an APHA color grade of 10 or less.
It is also desirable that filter elements for concentrated inorganic mineral acids be comprised of polymeric membranes. Such elements offer ease and economy of use, safety, excellent flow characteristics, and the ability to remove very small contaminants due to the high filtration performance of such membranes. Several fluoropolymers are known to be resistant to the corrosive actions of concentrated inorganic mineral acids. These include, for instance, (poly) vinylidene difluoride (PVDF) and (poly) tetrafluoroethylene (PTFE). Microporous membranes made from such fluoropolymers are currently used for the filtration of sulfuric acid. For example, PTFE microporous membranes are used for concentrated sulfuric acid filtration. These PTFE filters exhibit a high level of resistance to sulfuric acid and, if appropriately designed, an acceptable level of extractables, as indicated by the APHA color test. However, the media of these filters do not spontaneously and completely wet with sulfuric acid, and are not, therefore, in situ integrity testable in inorganic mineral acids. PTFE membranes do not wet with high surface tension fluids, such as concentrated sulfuric acid, due to the hydrophobic nature of these membranes. Consequently, flow of concentrated sulfuric acid through these filters cannot be initiated without first employing complex and dangerous pre-wetting procedures.
The current procedure used to wet fluoropolymeric filter media with a concentrated inorganic mineral acid, e.g., sulfuric acid, is time consuming, expensive, contaminates the acid, and is dangerous. The filter medium must first be pre-wet with a fluid having a low surface tension in order to overcome the medium's resistance to wetting with liquids having high surface tensions. Typical pre-wetting liquids are alcohols, such as ethanol or isopropanol. Once the membrane has been pre-wet the alcohol must be removed by flushing with deionized water. If the alcohol is not removed from the membrane, sulfuric acid will react with the alcohol resulting in the introduction of undesirable chemicals. After flushing with water, the membrane must be wet with a dilute acid by replacing the water in the membrane pores with a dilute acid solution. Flushing with a dilute acid prevents the build up of extreme heat (having the potential to damage the membrane) that occurs when concentrated sulfuric acid is mixed with water. The last step involves wetting the membrane pores with concentrated sulfuric acid. Only after this pre-wetting procedure is completed, which results in a great deal of waste, can the filter be utilized to remove particulates and contaminants from the sulfuric acid. These filters are not in situ integrity testable, however, because they do not spontaneously and completely wet with the acid.
In the microelectronics industry, contamination of processing fluids leads to high rates of defective product. As a result, the necessity for point-of-use or point-of-distribution filters which overcome the problems associated with filtering concentrated inorganic mineral acids has grown. Filter devices currently available for filtering concentrated inorganic mineral acids do not spontaneously and completely wet with the acids, and are therefore, not in situ integrity testable. Furthermore, some of the filters leach significant levels of TOC or ionic species (demonstrable as a high level of color transmission to the effluent).